fix[iOS]: app crash on open

[peterchinman]

tauri-plugin-app-events was crashing the app. Replaced the plugin with a native iOS lifecycle observer, which triggers a Rust FFI to trigger the javascript heartbeat check.

The iOS plugin called trigger() — a Tauri Plugin base class method — from a NotificationCenter observer.
Unlike Android, where the Plugin class provides lifecycle hooks (onResume()/onPause()) that Tauri calls when
its internal state is ready, iOS has no such hooks. The plugin used NotificationCenter as a substitute, but
trigger() assumes it's being called from within Tauri's managed lifecycle. When called from an external
notification callback, the Plugin's internal event dispatcher could be null, causing a null pointer
dereference (EXC_BAD_ACCESS at 0x0).

Note that there is a Tauri PR to solve the issue that this heartbeat method resolves. When that is merged we should use that handler for web content process termination instead of the heartbeat method.

[github-actions]

Preview: https://peter-fix-ios-crashy-crashy-9gpjjn.preview.macro.com/app (80bedee)

[seanaye]

storing the app handle in a oncelock is kinda weird. We should probably follow the recommended paradigm for calling from swift -> rust, because I think most plugins do not do this.

If this works though I guess you could merge it and I could look at this as a follow up

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.